A filled polyolefin composition is described based upon a polyolefin resin filled with a mechanico-chemically modified filler. The filler, which may be the usual filler material, is modified by comminution, by mechanically exposing fresh surfaces of the original filler particles in the presence of a polymer of a vinyl monomer. The vinyl monomer should have at least one polar group capable of bonding with the freshly exposed surfaces of filler material as they are formed and exposed during the mechanical operation. The process for the production of the modified filler is included.